1. Field of the Invention
This invention relates to an electrophotographic photosensitive layer, and more particularly to an improvement in the performance of an electrophotographic photosensitive layer through improving the composition of the resin binder.
2. Description of the Prior Art
As the photosensitive layer used in conventional electrophotography involving the steps of a uniform charging of a photosensitive layer formed on a support, toner developing and fixing to produce toner images, a variety of types of photosensitive layers are known in the art which include, for example, a photosensitive layer in the form of a coating film, which comprises a uniformly mixed system of finely divided photoconductive powders and a resin binder; photoconductive metals as represented by selenium; a photosensitive layer comprising a vapor-deposited film of an alloy; and a continuous film of a photoconductive organic polymer as represented by polyvinylcarbazole.
Of the above enumerated photosensitive layers, a photosensitive layer in the form of a coating film, which comprises a uniformly mixed system of finely divided photoconductive powders and a resin binder, is most generally used because of the relative ease of its preparation, its low cost, and the ease in improving its properties.
Electrophotography is now widely applied in practice in varied reprinters such as those commercially available under the trade names of Electrofax.RTM. (RCA) and Xerox.RTM. (Xerox Corporation) reprinters.
On the other hand, a processing method comprising forming an electrophotographic photosensitive layer on a material to be processed, marking, in an electrophotographic manner, machining information on the layer and conducting different machining operations on the material in accordance with the machining information is also employed in practice, known commercially as the Electro-Print-Marking (EPM.RTM.; Fuji Photo Film Co., Ltd.) process.
Also, as the photosensitive layer for use in the EPM.RTM. process photosensitive layers of the powder dispersion type are widely used. These layers comprise, predominantly, finely divided photoconductive powders, especially finely divided zinc oxide powders, and a resin binder. The reasons for this are that their preparation is relatively easy; that the production cost is low; that it is easy to improve their properties; that their photosensitivity is appropriate for practical use; and that white photosensitive layers are obtainable, etc.
In the EPM.RTM. process, different machining is conducted after electrophotographic marking. Machinings conducted include mechanical, discharge, or electrolytic machining. As a consequence, the photosensitive coating film layer formed on a support, which is also the material to be machined, must have a high degree of processing resistance. Thus, in machining on a milling cutter or in drilling with a drill, the photosensitive layer must not be softened by frictional heat generated between the cutting tool and the material to be machined; the photosensitive coating film must not delaminate from the material due to poor adhesion to the support; the photosensitive layer, should the layer be softened, must not become sticky, in order that chips from the support do not embed in and adhere to the softened areas of the layer which would otherwise impair the other areas of the layer; and that, in electrodischarge machining, when treated in an insulating liquid such as kerosene, the toner images marked on the photosensitive layer must not be impaired through swelling or dissolving. However, most resin binders used in electrophotographic photosensitive layers are of relatively low molecular weight, and, therefore, have lower softening points. Furthermore, since the proportion of the resin in the composition in the photosensitive layer is low and the photosensitive powders occupies a high proportion of the layer, the adhesion with the support cannot be expected to be too high.
Moroever, in the EPM.RTM. process, a photosensitive layer is formed on a support which is a material to be machined, on the spot where the material is machined. In usual cases, a paintlike coating composition comprising, predominantly a resin, photoconductive powders and a solvent, is applied onto a support having an unspecified shape and an unspecified dimension to form a photosensitive coating film thereon. As a consequence, after forming the photosensitive coating film formed on the support by coating, it is preferred for image formation to become possible as soon as possible. In general, immediately after the coating, that is, before the applied coating film has sufficiently dried, the charging capacity of the photosensitive layer is low and the charge retention thereof is also low. This results in a low charge potential and in a high speed of potential attenuation in the dark. For these reasons satisfactory images are not obtainable before sufficient drying has occurred. As drying proceeds the charging response improves and in some cases drying by heating becomes necessary. However, in practical use, a highpower drying device is not desired. In addition, when the photosensitive layer coated on the support is charged and imagewise exposed without sufficient drying, the charge dissipation is in general not complete in the exposed areas. As a result, fogging tends to occur upon toner development.
As described in the foregoing, especially in the EPM.RTM. process, photosensitive layers are desired in which the coating films have high machining resistance and produce satisfactory toner images by relatively simple drying treatment, within a short period of time after application.
In the case of photosensitive layers for use in reprinters, a long period of time lapses from the formation of the photosensitive layers to the actual use of them by the ultimate users, during which period of time sufficient drying is achieved and the electrical properties are enhanced. The matters as described above for the photosensitive layers for EPM.RTM. offer no problem in this case.
If it were to become possible, in the EPM.RTM. process, to conduct the image forming treatment within a short period of time after formation of the photosensitive layer, a lot of advantages would be achieved, for example, the flow of materials would not be retarded on the spot where the machining is actually carried out, and in marking treatment, automatic handling from carrying in to carrying out of the materials to be machined becomes possible.